Heating apparatus



May 6, 1952 G. E. ANDREWS HEATING APPARATUS Filed Maroh14, 194'? T M 00mm WC www. w

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Patented `May 6, 1,952

HEATING APPARATUS Glenn E. Andrews,

Andover, Mass., assignor to Raytheon Manufacturing Company, Newton,Mass., a corporation of Delaware Application March 14, 1947, Serial'No.734,568

1`Claim. l

This invention relates to heating apparatus, and more particularly to aheat-responsive controlling means for a microwave heating or cookingdevice.

An object of this invention is to provide a means, useful in conjunctionwith apparatus for heating dielectric bodies by alternating electricalenergy, for automatically controlling the supply of energy to the bodybeing heated.

Another object is to devise a means for automatically cutting off thesupply of alternating electrical energy to a dielectric body beingheated, when said body reaches a predetermined temperature.

A further object is to provide a device for microwave cookers whichautomatically shuts oi the microwave generator when the food beingcooked reaches a predetermined temperature, this device being capable ofmanual adjustment to vary said predetermined temperature.

A still further object is to devise a controlling means wherebydielectric bodies being heated may be brought to a certain predeterminedend or nal temperature, after which the energy is automaticallydecoupled from said body, this end temperature being attained regardlessof the initial temperature of the body, the size of the body, or therate of energy absorption of the body.

An additional object is to provide an automatic bodyheatresponsive,microwave energy controlling means for a microwave heating apparatus,which will not interfere with the absorption of microwave energy by thebody being heated and will not be affected by said microwave energy.

The foregoing and other objects of the invention will be best understoodfrom the following description of some exemplifcations thereof,reference being 'had to the accompanying drawing. wherein:

Fig. 1 is a schematic illustration of an arrangement whereby theinvention may be carried out;

Fig. 2 isa schematic illustration of a modied arrangement; and

Fig. 3 is a partial View, on an enlarged scale, showing a detail.

Now referring to Fig. 1, a hollow rectangular prismoidal enclosure or.cavity I is made of a suitable metal and has rather thin walls as shown;enclosure I is adapted to serve as the oven of the heating or cookingapparatus. A dielectric body 2 to be heated, which may be a container offood for example, is positioned in enclosure I, said body resting on thebottom wall of the ovenwhile the body is being heated. In order to allowaccess to the interior of the oven for placing the body 2 therein andfor removing said body therefrom, an opening 3 is provided in the frontwall ofthe oven I, this opening being closable by means of a hingedmetal door 4. When door 4 is closed, the enclosure I is entirely closed,except for the openings Il and I8 to be described hereinafter.

Numeral 5 generally designates an electrondischarge device of themagnetron type, which includes,` for example, an evacuated envelope 6,made of highly conductive material, such as copper, and provided with aplurality of inwardlydirected, radially-disposed anode vanes` 1. Thearrangement is such `that each pair of adjacent anode vanes forms,together with that portion of the envelope lying therebetween, a cavityresonator whose natural resonant frequency is, as is well known to thoseskilled in the art, a function of the geometry of the physical elementsmaking up the same. For an oven as above described, it is desirable thatthe dimensions of each such cavity resonator be such that thepredetermined wavelength of the electrical oscillations adapted to begenerated therein has a value which lies in .the microwave region of thefrequency spectrum. However, this invention is equally applicableinheating apparatus utilizing alternating electrical energy of anyfrequency useful for heating dielectric bodies by electrical energy..

Centrally located in envelope 6 is a highly `electron-emissive cathodemember 8, for example, of the well-known alkaline-earthmetaloxide type,said cathodemember being provided with .conventional means (not shown)for raising the temperature thereof to a level sufficient for thermionicemission.

The electron-discharge device or magnetron oscillator or radio-frequencyoscillator 5 is completed by magnetic means (not shown) for establishinga magnetic field in a direction transversely of the electron pathbetween the cathode and anode members thereof.

Anode or envelope 6 of the oscillator is connected, by a conductor 9,directly to the positive terminal of a suitable power source or directcurrent supply, while cathode 8 of the oscillator is connected, inseries with the normally-closed contacts I0 of a relay II, by aconductor 39, to one contact 42 of a switch I2, the other contact 4I ofsaid switch being connected by a conductor 40 to the negative terminalof said power source; switch I2 is normally closed to connect contacts4I and 42, as will be later described. It will be seen that, normally,oscillator 5 is connected to be energized from the power source, andthat by 'the opening o contacts IIJ or the opening of contacts 4I-42 theoscillator will be disconnected from the power source and therebydeenergized.

When energized, radio-frequency oscillator 5 delivers radio-:frequencyenergy to a hollow metallic waveguide I3 through a coaxial transmissionline I4 which is coupled to oscillator 5 by a loop I5. The centralconductor I6 of line I4 extends into the interior of waveguide I3 nearone end thereof, through an opening provided in the wall thereof, toserve as an exciting rod or exciting probe for said guide.

The end of guide I3 nearest this exciting rod is closed, `while theopposite end of said guide is fastened to the rear wall of enclosure Iand is open. The interior of guide I3 is placed in energy-transmittingrelationship with the interior of the cavity I by means of an apertureI'I provided in the rear wall of said cavity, this aperture being of thesame size and configuration as the interior of guide I3 and beingaligned with said guide to place the interior of said guide incommunication with the interior of cavity I.

When magnetron oscillator 5 is energized, the radio-frequency ormicrowave energy produced thereby is propagated down waveguide I3 andemanates from the open end thereof into the interior of enclosure I.This energy impinges on the body E! and is in eect absorbed thereby,resulting in heating or raising the temperature of said body.

Pursuant to this invention, an aperture I8 is provided in the top wallof enclosure or oven I. this aperture preferably being located in thecenter of said wall, being circular, for example, and having a diameterd. Extending outwardly or upwardly from the top wall of oven I, at rightangles thereto, and aligned with aperture I8, is a hollow metallic tubeI9 which has an internal diameter equal to d. The diameter d of apertureIand oi tube I3 is made such that for the wave mode present in enclosureI, the cutoff wavelength of tube I9, considered as a hollow waveguide,is below the wavelength of the oscillations generated by oscillator 5.The cutoff wavelength for hollow waveguides depends lon thecross-sectional dimensions of such guides, as is well-known to thoseskilled in the art.

Mounted adjacent the outer end of tube I9 is a heat-responsive orheat-sensitive device 25, which may be, for example, a sensitive bank orseries of thermocouples in a vacuum. Device 2l] is here represented asincluding a bimetallic hot junction 2| enclosed in an evacuated envelope22, and a bimetallic cold junction 23. The arrangement of tube I9 anddevice 2B with respect to body 2 is such that junction 2I is exposed tothe heat radiated from said body. In other words, the axis of tube I9 ispointed at the body and the inner end of the tube is exposed to thebody, so that thermocouple device 20 in eifect looks at the body.Junction 23 is maintained at a so-called fixed or reference temperature,and when the temperature of junction 2l is raised above that of junction23, a thermal electromotive force, direct in nature, is produced betweenthe output terminals 24 and 25 of the device 20.

As used herein, the phrase thermal electromotive torce has the meaningcommonly associated with it by those skilled in the science of physics.A current flows in a circuit composed of two metallic conductors ofdifferent materials if the temperatures of the two junctions aredifferent; in such a circuit there is an electromotive force dependenton thermal conditions and called a thermal electromotive force.

The electromotive force thus produced between terminals 24 and 25 isconnected as the input voltage of a direct current vacuum-tube amplifier26, and to this end terminal 24 is connected by means of a lead 21 toone of the input terminals 28 of said amplier, and terminal 25 isconnected by means of a lead 29 to the other input terminal 3S of saidamplifier. A lead SI connects one end of the operating winding 35 ofrelay I I to one of the output terminals 32 of amplifier 2S, while alead 33 connects the other end of said winding to the other outputterminal 34 of said amplifier.

Amplifier 26 has a variable gain, and this gain is such that normallythe output of said amplifier is insufficient to energize relay II; as aresult said relay is unenergized and contacts I0 are closed.

Switch I2, as shown in Fig. 3, may be mounted on the rear face of thedepending portion of the iront wall of enclosure I, adjacent to thehinge of door 4. The actuating rod of switch I2 is spring-biased to theposition indicated in dotted lines in Fig. 3, in which position thecontacts 4I and 42 of said switch are not connected to each other or areopen. The switch actuating rod is arranged to be forced, by the movementof door 4 from its dotted-line open position to its solidline closedposition, against the bias of the spring, to the position indicated insolid lines in Fig. 3, in which the contacts 4I and 42 of the switch areclosed or connected to each other. When door 4 is opened, the pressureon the switch actuating rod is released, so that said actuating rod ismoved to the right by the spring to open the contacts 4I and 42. Thus,when door 4 is closed, after body 2 is placed inside the oven I, thecontacts ll-42 of switch I2 will be closed, and when door 4 is opened toremove body 2 said contacts will be opened.

Assume, now, that a dielectric body 2, which is at some initialtemperature and which is desired to be heated to a predetermined finaltemperature, is placed in enclosure I and door 4 is closed. Switch I2 isclosed by the closing of said door and, since junctions 2l and 23 are atthe same temperature, device 25 produces no thermal electromotive forceand conditions are normal, being such that relay I I is unenergized andcontacts IB are closed. Therefore, the controllable circuit whichconnects oscillator 5 to the power' source is completed, and saidoscillator, being energized, produces radio-frequency energy which issupplied to the interior of enclosure I, where it impinges on dielectricbody 2 to heat the same. As body 2 is heated or as it rises intemperature, it radiates heat. This radiant energy in the form of heatfalls on junction 21 (since junction 2I is exposed to this radiant heatenergy of body 2), producing a rise in temperature at said junction andconsequently a direct thermal electromotive force between terminals 24and 25. This electromotive force is applied to the input of amplifier26, in such a direction as to tend to increase the output of saidamplifier. This thermal electromotive force is proportional to thetemperature of junction 2l, and, when it reaches a denite predeterminedvalue, will cause the output of amplier 26 to increase to a levelsuicient to energize relay II, opening its contacts IIJ, thus 5.deenergizing oscillator 5, thereby stopping Ithe supply ofradio-frequency energy to enclosure I and body 2. The aforesaid denitevalue of thermal electromotive force necessary to produce energizationof relay I I is Variable at will by-varying the gain of ampliiier 26,thereby varying the output level of the amplifier relative to the inputlevel thereof. The heat energy radiated from body 2 is proportional tothe temperature of said body, and the temperature of junction 2I isproportional to the radiant heat energy falling thereon, so that thethermal electromotive force produced between terminals 24 and 25 isproportional to the temperature of body 2. Thus, when body 2 reaches apredetermined temperature, oscillator is automatically deenergized andthe supply of radio-frequency energy to said body thereby stopped.

When magnetron oscillator 5 has been so deenergized, the door 4 may beopened and the body 2 removed. Opening of said door causes the contactsof switch I2 to be opened, so that even though body 2, which is now asource of radiant heat, be removed from the oven, thereby also removingthe thermal electromotive force from amplifier 2S and deenergizing relayII to reclose contacts I0, oscillator 5 will not be reenergizedunnecessarily, because of the series connection of the contacts ofswitch I2 and of contacts I0. The contacts of switch I2 are reclosed bythe reclosure of door 4 after another body 2 has been placed in the ovenI, to energize the oscillator 5 for the next heating cycle, contacts IIof relay II being closed at this time because of the absence of athermal electromotive force from thermocouple 20.

As explained above, the denite value of thermal electromotive force (andtherefore also the temperature of junction 2I and the temperature ofbody 2) at which relay II is energized to deenergize oscillator 5 isvariable by varying the gain of amplifier 26. Therefore, when body 2 isfood, the food can be heated to various selected temperatures byappropriately varying the amplifier gain. For instance, the food couldbe cooked rare, medium, or Well done.

The microwave energy of oscillator 5 is automatically cut off when andonly When body 2 reaches a predetermined temperature. This means that,when body 2 is food, such food is by this invention heated until itreaches the temperature at which it is cooked, regardless of its initialtemperature, its size, or its rate of energy absorption. If a presettiming device were used to control the application of radio-frequencyenergy to the dielectric body, rather than the device of this invention,various different initial temperatures of the bodies, different sizes ofthe bodies, or diiferent rates of energy absorption of the bodies mightresult in underheating or overheating of the bodies. However, with thesystem of the invention, such possible undesired end results areeliminated.

Since tube I9 is designed to make its cutoff wavelength below thewavelength of the radiofrequency oscillations of source 5, suchoscillations will not propagate down said tube as a hollow waveguide.Therefore, radio-frequency energy will not be transmitted away from theinterior of enclosure l to be lost for purposes of heating body 2, sothat the means of this invention will not interfere with the absorptionof energy by the body being heated. Also, since radio-frequency energyis not transmitted down tube I9, the heat-responsive device will not be6 a actuated or affected by such energy, but will be actuated by orsensitive to only the` radiant heat energy of body 2.

In ovens or enclosures of the above-described type, the air within theenclosure is not heated appreciably by the radio-frequency energy, sothat device 29 is not affected by the temperature of the-atmospheresurrounding the dielectric body, as would be the case with an ordinaryoven which relies on heat transmission by conduction of heat energythrough the atmosphere from the heat source to the body.

It is within the scope of this invention to make the aperture I8 of anyconvenient shape, element I9 having an opening therethrough of acorresponding configuration. For example, aperture I8 may be rectangularand element I9 may have a hollow rectangular cross-section, in whichcase the cutoff wavelength formulae for rectangular hollow waveguidesWould-be used to calculate the internal dimensions of portion I9 so asto make the cutoff wavelength of waveguide portion I9 less than thewavelength of the oscillations 0f oscillator 5.

Now referring to Fig. 2, in which similar elements are denoted by thesame reference numerals as in Fig. l, a modified structure for couplingthe heat-responsive device or thermocouple 20 to the enclosure I isdisclosed. In this modiflcation, a coaxial stub 3'I is provided, saidstub being a quarter-wavelength long and having an open lower end and ashort-circuited upper end. The open lower end of stub 3'I is alignedwith enclosure aperture I8, with the axis of the stub pointed at body 2and with the open end of the stub exposed to said body.

The shorted end of stub 31 has a relatively small aperture 38 therein,and the junction 2I of thermocouple 20 is placed directly above thisaperture, so that the thermocouple junction 2I is exposed to the heatradiated from body 2, the thermocouple thereby in effect looking at thebody 2.

Since junction 2 I is exposed to the heat radiated from body 2, athermal electromotive force will be produced between terminals 24 and 25of the thermocouple as body 2 heats, and this electromotive force may beutilized, as in Fig. l, to control the radio-frequency oscillator insuch a way as to stop the supply of energy to body 2 when the samereaches a predetermined temperature.

Due to the utilization of the shorted quarterwavelength coaxial stub 31in the above-described manner, no radio-frequency load additional tothat of the body 2 is imposed on the radiofrequency source, so that noradio-frequency energy is abstracted away from the dielectric body 2and, in addition, the thermocouple is effectively isolated from theintense radiofrequency radiations, so that the heat-responsive device orthermocouple 20 will not be actuated or affected by such radio-frequencyenergy.

Of course, it is to be understood that this invention is not limited tothe particular details as described above, as many equivalents willsuggest themselves to those skilled in the art. For example, it may inmost cases be preferable to put the thermocouple tube or stub in theside wall of the oven, rather than in the top wall as shown, in order toprevent the entrance of cooking vapors, steam, etc. into the tube orline leading to the thermocouple. Various other variations will suggestthemselves. It is accordingly desired that the appended claims be givena broad inter- 7 pretation commensurate with the scope of this inventionWithin the art.

What is claimed is:

In combination, a metallic enclosure adapted to serve as an oven, anoscillator for supplying electromagnetic wave energy to the interior ofsaid enclosure to heat a dielectric body positioned in said enclosure, acontrollable circuit for energizing said oscillator from a power source,means for generating a thermal electromotive force, said means beingexposed to the heat radiated from said body and being cou-pled to saidoven in such a way that said means is isolated from said wave energy,and means for utilizing said electromotive force to control said circuitto deenergize said oscillator when said body reaches a predeterminedtemperature.

GLENN E. ANDREWS.

REFERENCES CITED UNITED STATES PATENTS Name Date Wallis et al. Oct. 14,1919 Number Number Name Date 1,981,631 Northrup Nov. 20, 1934 2,233,788Lewin Mal. 4, 1941 2,275,265 Mead Mar. 3, 1942 2,279,525 Rogers Apr. 14,1942 2,364,526 Hansell Dec. 5, 1944 2,370,161 Hansen Feb. 27, 19452,386,966 MacMllin Oct. 16, 1945 2,448,008 Baker Aug. 31, 1948 2,495,415Marshall Jan. 24, 1950 2,495,435 Welch Jan. 24, 1950 2,498,720 Wild etal Feb. 2B, 1950 2,500,752 Hanson et al. Mar. 14, 1950 2,526,226 GrossOct. 17, 1950 OTHER REFERENCES Stack: Vacuum Thermocouples of theRadiation Type, General Electric Review, vol. 42, No. 8, August 1939,pages 365 and 366.

Hutcheson: Electronic Torch, The Welding Engineer, December 1945, page90.

Walsh: Homemaking, The Washington Daily News, October, 9, 1946, page 38.

Magnetron Adapted to Cooking Purposes, Electrical Engineering, December1946, page 591.

